Radiant energy resulting from infrared emission by radiating surfaces has long been used to dry or cure coated objects. Heat energy transferred to a radiating surface by convective, contact, or radiative heating can in turn be radiatively transferred to the coating of an object, speeding the natural drying process that hardens the coating on that object. An example of convective transfer of heat to a radiating surface for the purpose of drying coated objects is found in Best, U.S. Pat. No. 4,546,553 in which opposed curved walls direct infrared radiant heat against painted objects passed through an oven chamber. The walls of the oven chamber are heated by directing turbulent air against the inside surfaces of the curved wall, causing the curved walls to heat and thereby radiate increased amounts of infrared heat into the oven chamber. This apparatus has the disadvantage in that the surfaces of objects placed within the oven chamber differentially heat up to a desired temperature because each area of the object generally receives a varying amount of incident infrared energy according to its particular distance and its surface orientation in relation to the radiating wall. The coating on the object is therefore heated at different rates, adversely affecting the drying process.
One attempt to minimize the effects of differential heating of objects placed in a drying oven has been disclosed in Best, U.S. Pat. No. 4,785,552. Using a radiant wall heating oven similar to that previously described, Best '552 additionally controls the equilibrium temperature of the surface of an object in an oven chamber through the use of induced air movement within the oven chamber. Air having a lower temperature than the temperature of the curved walls of the oven chamber is circulated in a desired direction through the oven chamber to cool selected portions of the object so that the temperature of the object remains constant at all points on its surface, ensuring the even drying of the coating on the object. However, such an apparatus for controlling the temperature of the object often requires highly filtered air, precise positioning of multiple blowers to circulate air and a detailed knowledge of the amount of convective transfer of heat from the object to the cooler air.
It is therefore an object of this invention to provide an apparatus for controlling the temperature of an object placed in a drying chamber of a radiant wall drying oven.
It is a further object of this invention to control the flux of infrared radiation emitted by selected portions of an infrared radiating wall of a drying chamber of a radiant wall drying oven.
Yet another object of this invention is to provide an apparatus having one or more control dampers which regulate the amount of heated air contacting an absorbing surface of a radiant wall of a radiant wall drying oven.
Accordingly, this invention comprises a radiant wall drying oven that includes a first wall and a second wall situated in spaced apart relation to define an air conducting passageway. Heated air is supplied to contact the first wall, and a selected amount of heated air is allowed to pass through inlets into the air conducting passageway. Valves, positioned at a predetermined site relative to the second wall, control the amount of heated air contacting a portion of the second wall. Since the temperature of that portion of the second wall controls the amount of radiant heat emitted by that area of the second wall, the amount of radiant heat directed against the object is regulated.
In preferred embodiments the second wall at least partially defines a drying chamber into which objects can be individually placed in a batch process, or may be alternatively conveyed by a conveyor in a continuous process. The drying chamber can be pneumatically sealed to prevent the introduction of dust, moisture, or other substances that can detrimentally affect the drying or curing process.
Heated air can be supplied to contact the first wall by the combination of a heater for heating air, a blower for propelling the heated air toward the first wall, and a first conduit for channelling the heated and blown air to contact the first wall. The heater can be any device that acts to heat air to a desired temperature, and may be gas-fired or oil-fired. A blower suitable for impelling the heated air into the first conduit can be a propellor or other type fan.
Air inlets may constitute apertures defined within the first wall at predetermined sites. Heated air blown by a fan or other impellor through the first conduit contacts the first wall and may only enter the air passageway defined by the space between the first and second walls through these apertures. As a consequence, those portions of the second wall that are initially contacted by the heated air passing through the apertures will be most strongly heated, and consequently radiate increased amounts of infrared heat relative to those portions of the second wall that are not initially contacted by the heated air flow.
Heated air that has contacted the second wall will be cooled by the transfer of heat energy to the second wall. The cooled air can be exhausted from the drying oven, or in preferred embodiments, can be directed by a second conduit back toward the heater for heating air. Recirculating the air in this manner has has the advantage of reducing the heat required to heat the air to a desired temperature because the air, although cooled following contact with the second wall, is still significantly hotter than air at room temperature. Less heat energy is therefore required to raise the temperature of recirculated air to a desired temperature than is required for heating fresh air to the desired temperature. However, complete recirculation is generally, not advisable, since fresh air should be added to the recirculating system to replace air escaping the recirculating system and replenish the loss of oxygen during combustion processes in the heater. Also, air containing combustion products such as carbon dioxide and carbon monoxide should be exhausted to prevent the reduction in heater efficiency by stifling the combustion process.
The emission of radiant energy into a drying chamber by the second wall can be precisely controlled by the utilization of damper plates fitted over selected apertures in the first wall. By opening or closing the damper plates to a greater or lesser extent, the amount of air passing through the apertures in the first wall and convectively transferring heat energy to the predetermined portions of the second wall can be regulated. In preferred embodiments, the damper plates are fixed on the first wall to permit sliding movement of the damper plate, blocking by a desired amount the free flow of heated air through the apertures, and thereby controlling the amount of radiant heat emitted by selected portions of the second wall into the drying chamber. Such sliding dampers can be manually or automatically positioned as desired. If positioning of sliding dampers is manual, in preferred embodiments access to the sliding dampers is provided by an access door in the first conduit that permits access to the sliding dampers fixed on the first wall. The extent to which the sliding dampers block the flow of heated air through an aperture can also be determined automatically, using thermocouples or other temperature sensitive devices that provide feedback to art-recognized devices for controlling the positioning of the sliding dampers. An apparatus used in this manner can automatically control the temperature of selected portions of the second wall by opening the sliding dampers when the temperature drops below a desired predetermined value, and closing the sliding dampers when the temperature rises above the desired value.
One advantage of the invention is the greatly improved control over the temperature of selected portions of a radiating wall of a drying chamber, and consequent control over the surface temperature at all points on an object in that drying chamber. Unless an object has a surface that exactly corresponds to the radiating surface of the radiating walls of a drying chamber, the amount of incident infrared radiation impinging on the object will vary over the surface of the object. Since the rate of drying of an object coated with a liquid in a radiant drying chamber is a function of the amount of the incident radiant infrared energy, the coating on the object may differentially dry, causing adverse effects such as wrinkles or creases in the coating. The present apparatus minimizes these adverse effects by regulating the amount of incident radiant energy through the use of control dampers that control the amount of heated air contacting selected portions of the radiant wall. For example, consider an object that has a surface region closely approaching the radiating second wall toward the bottom of the object, and has a second surface region more distantly located from the radiating second wall toward the object's top. The even drying of such an object may be promoted if control dampers located on the first wall across from that portion of the radiating surface of the second wall located near the bottom of object are nearly closed to minimize the heat radiation of the second wall, and the dampers are more widely opened to increase the amount of heat transferred to those portions of the second wall positioned to radiatively heat the more distant regions of the object's surface. By appropriate positioning of apertures and control dampers, a wide variety of objects having various shapes can be evenly heated in a radiant wall drying oven according to this invention.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.